Currently, much industrial fermentation involves the manufacture of ethanol for either chemical or fuels use. For use in fuel, butanol has advantages as compared to ethanol, namely butanol has a lower vapor pressure and decreased solubility in water.
An advantageous butanol fermentation process would encompass a complete, or substantially complete, conversion of sugars to butanol without reaching a butanol titer above a threshold of butanol tolerance that causes the rate of butanol production to fall below an undesirable predetermined rate. While it may be possible to limit sugar loadings to a level whereby batch fermentation does not require operation at a butanol concentration above the tolerance level, this approach has disadvantages because limited sugar loadings result in dilute solutions that are themselves economically undesirable to process. Therefore, there is a need for a process by which levels of butanol are limited in a fermentation at or below the tolerance level whilst sugar loadings are not limited by considerations of the tolerance level.
One means by which a butanol producing fermentation process might be made more efficient would be to remove the butanol as it is being formed from the fermentation medium (broth), so that the tolerance level of the butanol producing micro-organism is not reached, allowing high loading of sugar to be charged to the fermentation vessel. Such an “in situ product removal” or “ISPR” process is described in PCT International Publication No. WO2009/079362 A2.
ISPR processes for fermentation products are also described in the Roffler dissertation (Roffler, Steve Ronald, “Extractive fermentation—lactic acid and acetone/butanol production,” Department of Chemical Engineering at the University of California at Berkeley, 1986). Roffler describes a process whereby a liquid stream from a fermentation vessel is passed to a separate vessel which is held under vacuum. However, the method described in Roffler necessitates further processing of the resulting vapor stream. Because an industrial fermentation relies on microorganisms, such processing must consider temperature constraints relative to the microorganisms.
To operate at acceptable temperatures, consideration must be given to costs and practicalities of cooling or operation under vacuum. The costs associated with removal of heat within a chemical process can be a function of the plant location and also the time of the year. In many geographic areas, it is not possible to guarantee cooling to be available or practical at the temperature at which heat needs to be removed from the vapor stream.
Providing chilled water to the heat exchanger by which condensation is carried out significantly increases the cost of the cooling medium. An alternative would be to compress the vapor stream to a higher pressure to allow the condensation to be done against cooling water year round, but this too entails significant cost because of the low density of the initial vapor passing to the machine. Processes described which use lithium bromide for absorption of ethanol and water vapors may not be adequate for absorbing carbon dioxide or higher alcohols of a vapor stream.
In addition, with whatever method is used, there will be a residual gas stream (due to the solubility of CO2 in the fermentation broth) that must be compressed before discharge to the atmosphere. The residual gas stream will comprise CO2. While vacuum flashing represents an effective means by which butanol can be removed from a fermentation process, there is a need for advances in the processing of the resulting low pressure vapor stream containing the product.